Size Selectivity of Passive Fishing Gear: A Correction for Encounter Probability Applied to Gill Nets

The probability of catching a fish in a gill net may be separated into two components: (1) the probability of the fish encountering the net and (2) the probability of the fish being caught and retained in the net. We consider the probability of encounter to be directly proportional to the distance travelled by the fish during the sampling period. This distance will increase with fish size if different-sized fish swim for the same amount of time because swimming speed increases with fish size. Routine swimming speed measured in the laboratory for three size-classes of bloater (Coregonus hoyi) increased with length to the 0.8 power. Corrections for encounter probability were incorporated in gill net selectivity calculations for samples of cisco (Coregonus artedii), a species closely related to bloater. These corrections can significantly increase the proportional estimates of smaller relative to larger animals in the estimated population structure. The approach should also be applicable to other passive fishing gear, such as longlines and set nets.